Unstable power quality usually causes a damage, a crash or a data loss of the computer and all kinds of precision instruments, and thus, an uninterruptible power system gradually becomes an indispensable auxiliary equipment for the computer and all kinds of precision instruments. The so-called uninterruptible power system, as implied by the name, is the equipment for supplying emergency power when the external power is cut off or abnormal, and it is generally box-shaped and composed of a battery, a monitor software and a relay. Certainly, the main structure of the uninterruptible power system is the battery, whose power is saturated in peacetime. Once the uninterruptible power system detects that the external power is disconnected or the voltage is abnormal, the power source will be switched to the battery in an extremely short time, and the DC power in the battery will be converted into the AC power for loading. Therefore, through the backup battery, there will be a buffer time of several minutes, even several hours, for completing the current works and shutting off the system in accordance with a normal procedure so as to avoid data loss or system damage.
For the uninterruptible power system, the battery is an important element. In most uninterruptible power systems, there usually exists an external and independent battery box for extending the battery discharging time. However, because there is a possibility that the battery has a leakage that may cause a ground fault, or the terminal of the battery has a short fault to the ground owing to an inappropriate transportation or incorrect assembling, the battery might be damaged in a minor situation, or seriously, the battery might be burned, which causes a security problem or destroys the uninterruptible power system and thus results in the power failure.
Please refer to FIG. 1(a), which is a schematic view showing a circuit framework of a conventional uninterruptible power system. As shown in FIG. 1(a), the core elements of the uninterruptible power system include an input power source 10, a rectifier 11, a battery box 12, a battery 121 and a converter 13. When any battery 121 in the battery set contacts with the battery box 12 due to a leakage thereof, a current loop will be formed between the battery and the ground because the case of the battery box 12 has been equipment grounded. Therefore, a spark and overheat might be occurred at the leakage, so that the case of the battery 121 might be overheated and burned. Although the battery 121 can be positioned in a specially made insulation box for avoiding the leakage from contacting with the case, it is still undependable.
Regarding the ground fault problem of the battery 121 in the battery box 12, JP patent application No. 7-146321 published on Jun. 6, 1995 proposed a protecting manner that a leakage breaker 14 is mounted between the input power source 10 and the rectifier 11 so that when the detector 15 detects an abnormal input power source, it will generate a signal to trigger the leakage breaker 14, as shown in FIG. 1(b). However, because the sensitivity of the ground leakage breaker 14 is still a problem, there is a possibility that an unusual or abnormal error might not be detected by this manner, and further, the cost thereof is higher.
Another battery ground fault detecting method is disclosed in U.S. patent publication No. U.S. 2003/0030440A1 published on Feb. 13, 2003, which employs a voltage dividing resistor for detecting whether a ground fault is occurred or not. As shown in FIG. 1(c), when a ground fault is occurred in the battery 121, the detector 15 will send a signal to the microcontroller 17 of the uninterruptible power system in response to the abnormal voltage of the resistor 16, so that the microcontroller 17 may alert the system or execute a procedure properly. However, because there exists impedance in the electrolyte when the leakage ground fault is occurred in the battery 121, this method still cannot detect the battery ground fault ranged in a middle magnitude owing to the precision problem.
Please refer to FIG. 1(d), the detecting method illustrated therein employs an isolation transformer 18 added at the input terminal of the input power source 10 for avoiding a formation of current loop caused by a ground fault of the battery 121. However, because the capacity of this isolation transformer 18 has to be identical to that of the uninterruptible power system, the cost and installation space must be increased. Therefore, in the viewpoint of cost-benefit, this detecting method is not a best processing way.
According to the above descriptions, it is obvious that the application of the uninterruptible power system is focused on improving the reliability of power supply, and thus, the uninterruptible power system should own the ability of detecting all kinds of faults and executing a corresponding procedure properly. Certainly, the existence of undetectable fault will reduce the reliability of the uninterruptible power system. Therefore, it is desired to develop a detecting circuit that can overcome the defects in the prior arts and detect the battery ground fault immediately.